There is often a need to cut a part that includes a substrate with a coating thereon. One example is an optical component having a substrate such as a lens, with a coating on one or both side surfaces of the lens. The coating may be an antireflective (AR) coating, a filter coating that passes some wavelengths and reflects other wavelengths, or a protective coating that helps avoid scratches or other physical damage to the substrate. Sometimes the substrate can be cut before it is coated, but this is not always possible, and in some cases it is not cost-effective to cut the substrate and then coat it. Consequently, it is sometimes necessary to cut the part at a point in time after the coating has been applied to the substrate.
A common problem is that the coating is often mechanically tougher than the substrate, and will tend to hold together and pull away from the substrate, rather than remain bonded to the substrate and be cut apart. For example, where a hole is to be drilled through a part, the drill first makes contact with the coating. The drill exerts not only a downward force on the coating, but also a twisting force. In some cases, the coating will hold together despite the force, and an irregular portion significantly larger than the intended hole will be pulled off the substrate. Alternatively, when a coating is cut with a saw, the coating will be pulled sideways where the saw contacts the coating, and irregular portions of the coating can be pulled off the substrate. Sometimes the damage is sufficiently significant that the part must be scrapped, thereby reducing the effective yield of the manufacturing process, and increasing the average cost per completed part. Consequently, while existing cutting techniques have been generally adequate for their intended purposes, they have not been entirely satisfactory in all respects.